The invention relates to a device for comparing an optical signal wavelength or wavelengths with at least one nominal wavelength, the said device including a phasar with a waveguide grating.
The invention also relates to a system for monitoring a tuneable optical source serving notably to generate the carrier waves signal.
The comparison is made with a view to the wavelength monitoring of the optical sources used for effecting wavelength multiplexing in telecommunications networks. The increase in the density requirements of the transmission channels of networks using wavelength multiplexing accentuates the importance of a rapid monitoring of the wavelengths of the optical sources and their stability; these wavelengths can undergo drift due to the aging of or temperature variations in optical sources.
Nominal wavelength means the emission wavelength imposed on the optical source. Nominal wavelengths correspond typically to values standardised by the International Telecommunications Union (ITU). It is possible to use lasers as optical sources. Input port (or output port) generally means an input guide (or output guide).
Drift correction consists of the wavelengths emitted by the optical sources designated as xcexi corresponding to the nominal wavelengths designated as xcexei.
Amongst the different types of known passive multiplexers, consideration will be given hereinafter to those which use an angular dispersion element such as an etched diffraction grating or a grating formed by waveguides, connecting two star shaped couplers. Hereinafter this type of multiplexer (referred to in English as xe2x80x9cPhased-Arrayed Waveguide Grating Multiplexerxe2x80x9d or xe2x80x9cAWGxe2x80x9d) will be referred to as a phasar.
One example of a device for the wavelength monitoring of optical sources of a phasar is proposed in the article xe2x80x9cIntegrated real time multi-channel wavelength monitoring circuit using phased-arrayed waveguide gratingxe2x80x9d by S. Zhong et al., OFC""99, pp. 30-32. This article describes the implementation of wavelength monitoring for a phasar: the comb of N wavelengths to be monitored and multiplexed is duplicated. One comb is used for the wavelength monitoring, the other for the useful part to be multiplexed.
Another example of a device for the wavelength monitoring of the optical sources of a phasar is proposed in the article xe2x80x9cA wavelength matching scheme for multiwavelength optical links and networks using grating demultiplexersxe2x80x9d by F. Tong et al, IEEE Photonics Technology Letters, Vol 7. No 6, June 1995. In the solution proposed, two transmission channels are dedicated to the comparison of the wavelengths.
In the article xe2x80x9cFabrication of multiwavelength simultaneous monitoring device using arrayed-waveguide gratingxe2x80x9d, Electronics Letters, Mar. 14, 1996, Vol. 32, No. 6, the authors K. Okamoto et al, describe a device making it necessary to take off some of the N input signals in order to reintroduce them into the phasar and to compare at the output the N corresponding signals detected on each side of the N principal output signals. In addition to a few losses caused by taking off some of the N input signals, this solution requires duplicating the input signals, an operation which is tedious to perform.
These devices also have black ranges for which wavelength monitoring cannot be achieved.
The purpose of the invention is to propose a solution which does not have the drawbacks mentioned above. It also makes it possible to widen the wavelengths monitoring ranges of the signals transmitted.
The invention applies to the monitoring of the wavelength of discreet lasers or tuneable lasers.
A detailed analysis of the functioning of the type of phasar envisaged in the invention and depicted in FIG. 1 shows that, for each wavelength value xcexi of the emission laser i, i varying from 1 to N, the optical wave constituting the input signal is coupled to an input guide GEi and undergoes the following operations:
diffraction in an input coupler CE, mathematically represented by the Fourier transform of the signal undergoing the diffraction, each guide Gj situated at the output surface SCE of the coupler receiving part of the diffracted wave,
phase shifts in a grating R of M guides Gj with variable optical paths, j varying from 1 to M, situated between the coupler CE and a coupler CS, the optical path njLGj travelled in a guide Gj being expressed as a function of the refractive index nj of the guide Gj and its length LGj; the phase shifts producing, at the output of the grating R of guides, interferences which are constructive in a direction dependent on the wavelength,
a focusing, on the output surface SCS of the coupler CS, of the constructive interferences of the waves issuing from the guides in the grating R.
The phasar according to the invention is designed so that at least two interference orders corresponding to two focal points form on the output surface SCS of the coupler CS for N given wavelengths, that is to say for the N wavelengths of the optical sources. The multiplexed signals resulting from the constructive interferences at these two orders are used for the wavelength monitoring.
More precisely, the object of the invention is a device for comparing an optical signal wavelength value or values with at least one nominal wavelength value xcexei, the said device including a phasar with a grating R of guides Gj, provided with N monitoring input ports GEi associated respectively with N nominal wavelength values xcexei, so that any optical signal having one of the said nominal wavelength values and being applied to the port associated with this nominal wavelength value focus at two focal points corresponding respectively to two interference orders of the grating R, the said phasar including two monitoring output ports A, B placed respectively about the said focal points, principally characterised in that the said phasar is designed so that the transmission function representing the variations in transmission between an input monitoring port and an output monitoring port as a function of the wavelength has a representative curve substantially triangular in shape about the nominal wavelength value associated with the said input port.
Concerning the practical embodiment design, an appropriate choice of the positions of the monitoring ports makes it possible to optimise the monitoring function. For this, use is made of the transmission functions TAi and TBi of the phasar defined respectively as the ratios of the optical powers present respectively at the outputs A and B to the optical power of the signal applied at the input port GEi, these power ratios being a function of the wavelength of the applied signal.
According to one embodiment characteristic of the invention, the optical paths njLGj of the guides Gj in the grating R of the phasar are adjusted so that the curve representing the said transmission function has a substantially triangular shape.
The positions of the output ports A and B are advantageously adjusted so that the difference between the two transmission functions TAi and TBi of the phasar for the output ports A and B, known as the discrimination function Di, is zero for the N nominal wavelength values xcexei and bijective as a function of the wavelength around the nominal value of each wavelength. The bijective character means that, to each wavelength value taken around each of the nominal values, there corresponds a single value of the discrimination function Di and vice-versa.
Another object of the invention is a device as described above in which the phasar comprises a common input port GElaser and N demultiplexing output ports associated respectively with the said N nominal wavelengths and such that any optical signal applied to the said common port and having one of the said nominal wavelengths is focused on the demultiplexing port associated with this wavelength and in that the said demultiplexing output ports are coupled respectively to the said monitoring input ports associated respectively with the same nominal wavelengths.
To allow an electronic processing of the discrimination function Di, the two output ports A and B are coupled to photodetectors.
The invention also concerns a system for monitoring a tuneable optical source Li, principally characterised in that it includes a comparison device as described in the previous two paragraphs and control means CM for monitoring the wavelength of an optical source coupled to the said common input port GElaser as a function of the electrical signals supplied by the said photodetectors.